1. Field of the Invention
The present invention relates to a temperature sensor having a temperature-sensing element, such as a thermistor element or a Pt resistor element.
2. Description of Related Art
A conventionally known temperature sensor for detecting the temperature of, for example, exhaust gas of an automobile is described in Japanese Patent Application Laid-Open (kokai) No. 2000-266609.
As shown in FIG. 6A, such a conventional (i.e., prior art) temperature sensor 500 includes a thermistor element 502 and a sheath member 506. The thermistor element 502 includes a thermistor sintered-body 503 and element electrode wires 504. The sheath member 506 is configured such that sheath core wires 508 are insulatively held in a sheath tube 507. The element electrode wires 504 extending from the thermistor sintered-body 503 are joined, by laser spot welding, to sheath core wires 508 extending from the front end of the sheath tube 507. A resultant joint portion 510 and the thermistor element 502 are accommodated within a metal tube 512 made of a stainless steel alloy. In order to hold the thus-accommodated thermistor element 502 and other components, a space enclosed by the metal tube 512 is filled with a holding material, such as cement 514 formed of a heat-resistant oxide (Al2O3 (alumina) or the like).
The temperature of exhaust gas or the like varies in a wide range from a temperature in a low-temperature zone, such as about 0° C., to a temperature in a high-temperature zone, such as about 1,000° C. Thus, the temperature sensor 500 is used to detect a temperature in such a wide range. Accordingly, the conventional temperature sensor 500 is exposed to an environment of repeated cooling/heating (low-temperature/high-temperature) cycles.
When the temperature sensor 500 is cooled quickly from a high temperature to a low temperature, cooling starts from an outer circumferential or enclosing member; specifically, from the metal tube 512 (e.g., a closed-bottomed tube). The metal tube 512 is made of a stainless steel alloy; the cement 514 charged into (i.e., filling) the space enclosed by the metal tube 512 is of alumina; and the stainless steel alloy is higher in thermal expansion coefficient than alumina. Accordingly, as shown in FIG. 6B, when the metal tube 512 begins to contract as a result of cooling, the contraction of the cement 514 fails to follow that of the metal tube 512. As a result, a front end portion (bottom portion) of the metal tube 512 presses the cement 514 in the direction of arrow A. Accordingly, the front end face of the thermistor element 502 (more specifically, the thermistor sintered-body 503) held in the cement 514 is also pressed toward the sheath member 506 (rearward). When the thermistor element 502 is pressed toward the sheath member 506, shearing stress is applied to the joint portions 510 between the element electrode wires 504 and the sheath core wires 508 as illustrated by arrows B.
When shearing stress is repeatedly applied to the joint portions 510 between the element electrode wires 504 and the sheath core wires 508 in response to repeated cooling/heating cycles, in the worst case, the joint portion 510 is broken, resulting in failure to obtain a detection output (i.e., signal) from the thermistor element 502.